Smoothing the way for William

‘Families want solutions that incorporate familiar, everyday equipment’
William used to fall three to four times a day.

“I might just be standing there, and my (prosthetic) knee would give out,” says the 12-year-old.

That instability is a thing of the past thanks to a new prosthetic knee William is testing for Bloorview.

This knee automatically locks when William extends his leg – preventing falls that occur with other knees that require kids to place weight on their foot before stabilizing.

“We wanted a system that would be safer,” says Jan Andrysek, the rehab engineer at Bloorview who designed it.

Traditionally, prosthetic knees locked when a child placed weight on the foot as it touched the ground. They then unlocked – allowing the knee to bend – when the child pushed off the toes and swung the leg forward. However, most also required children to consciously extend their hip muscles when they placed their foot down.

By locking as soon as the leg is extended, Jan’s knee “allows kids to pay attention to what they’re doing – rather than worrying about whether their leg is locked,” Jan says.

With the new device, leg straightening activates a spring that pushes a pin into a hole that secures the knee before weight is placed on the foot. When the child pushes off his toes, that force is amplified through a lever mechanism that removes the pin, unlocking the knee. This mechanism – similar to that used to open and close a door – also met another important design criterion: it was simple and therefore small enough to fit even a four-year-old.

Instead of relying on six or more main components, Jan’s knee uses only four. That means it’s about two inches shorter than its main competitor.

Jan’s knee is also designed to speed the time it takes the leg to swing forward. Traditionally, “the faster a child walks or runs, the longer it takes for the prosthetic leg to swing forward into the position where weight can be placed on the foot again,” Jan says. Jan’s knee has a spring that pushes a roller against the top part of the knee, storing energy as it compresses while the knee flexes. That energy is then released to help propel the leg forward faster.

Jan’s knee has evolved through three prototypes and “William has stuck with me for over two years, giving feedback that was invaluable,” Jan says. The knee was conceived as a masters project while Jan studied in a University of Toronto program that combines mechanical and biomedical engineering. It will now undergo further field testing.